As electronic devices increasingly become portable, advances must be made in energy storage systems to enable such portability. Indeed, it is often the case with current electronic technology that the limiting factor to portability of a given device is the size and the weight of the associated energy storage device. A small energy storage device, such as a battery, may be fabricated for a given electrical device but at the cost of energy capacity. Conversely, a long lasting energy source can be built but it is often too large or too bulky to be comfortably portable. The result is that the energy source is either too heavy or does not last long enough for a particular user's application.
Numerous different battery systems have been proposed for use over the years. Early rechargeable battery systems included lead acid, and nickel cadmium (NiCad), each of which has enjoyed considerable success in the market place. Lead acid batteries are preferred for applications in which ruggedness and durability are required and hence have been the choice of automotive and heavy industrial settings. Conversely, NiCad batteries have been preferred for smaller portable applications. More recently, nickel metal hydride systems (NiMH) have found increasing acceptance for both large and small applications.
Notwithstanding the success of the foregoing battery systems, other new batteries are appearing on the horizon which offer the promise of better capacity, better power density, longer cycle life, and lower weight, as compared with the current state of the art. The first such system to reach the market is the lithium ion battery, which is already finding its way into numerous consumer products. Lithium polymer batteries are also receiving considerable attention, although they have not yet reached the market.
Lithium batteries in general include a positive electrode fabricated of, for example, a transition metal oxide material and a negative electrode fabricated of an activated carbon material such as graphite or petroleum coke. New materials for both electrodes have been investigated intensely because of the high potential for improved energy density. To date, however, most of the attention has been focused on the transition metal oxide electrode.
Activated carbon electrode materials are routinely prepared by using difunctional monomers as polymer precursors. Examples of such precursors include resins of furfuryl alcohol, phenyl, formaldehyde, acetone, furfuryl or furfuryl alcohol-phenyl copolymers. Other precursors include polyacrylonitrile, and rayon polymers, as disclosed in Handbook of Carbon, Graphite, Diamond and Fullerenes, Hugh O. Pierson, Noyes Publications, Park Ridge, N.J., (1993). Materials that result from these processes are typically characterized by relatively low yields, as well as high cost and low capacity.
In addition to these considerations, it is desirable to employ readily available and renewable precursor materials, particularly those with a relatively high char yield, so as to yield an amorphous carbon material. It is noted that otherwise promising candidates among both synthetic and natural classes of materials melt during pyrolysis at temperatures well below the cure or thermoset point, resulting in viscous melts that foam upon the production of offgases prior to curing during carbonization. The resulting numerous pores harden into permanent morphological structure during heating, with attendant decrease of maximum electrochemical performance of the carbon product.
Accordingly, there exists a need for improved carbon materials for use in electrochemical cell applications.